The present invention is directed to an improved electrochemical process for oxidizing organic compounds and an effective means for regeneration of the spent oxidant. More specifically, the invention described and claimed herein requires the use of an organicsulfonic acid solution having certain thallium(III) organosulfonates dissolved therein as fully described hereinbelow.
The products achieved by the present invention have been previously formed to one degree or another by a variety of processes which can be classified as chemical or electrochemical. Various known oxidizing agents have been used to cause chemical oxidation of organic compounds. Oxidation has also been achieved by direct electrochemical oxidation, normally in the presence of an acidic electrolyte solution, as described in U.S. Pat. Nos. 4,298,438 and 4,354,904. Indirect electrochemical oxidation has been conducted in which the oxidant is electrochemically generated and then used to oxidize the organic substrate. Indirect electrolytic oxidation is discussed by Norbert Ibl et al at page 45 et seq. in Electro-organic Synthesis Technology, 75, No. 185 (1979) which teaching is incorporated herein by reference.
Compounds which are known to be capable of acting as an indirect oxidant include transition metal salts, particularly the metals of cobalt, chromium, manganese, iron, lead, silver and cerium. Because regeneration of the spent metal to its higher oxidation state is not always highly effective and/or other insoluble salts, such as oxides, etc., are formed, those skilled in this art tend to use the salts of chromium, manganese, cobalt, iron or lead as these salts are less expensive and replacement of spent materials do not greatly detract from the economics of the process. However, each of these metal ion oxidants have certain properties which cause them to make the oxidation process ineffective. For example, chromium ions give poor selectivity towards the desired products, cerium and manganese salts are believed to have low solubility of the oxidized and/or reduced ions in acidic solutions, the higher oxidation states of silver, cobalt and lead ions are not very stable and, in the case of iron, is not very reactive. Indirect electrochemical oxidation has been further complicated by the properties of the anion specie present. For example, certain anions (e.g., chloride, nitrate, perchlorate) are highly reactive with the organic substrate producing by-products or conditions which preclude their use on a commercial scale. Other less reactive anions (e.g., sulfate, acetate, fluoride, boron fluoride, silicon fluoride) generally form salts of low solubility, inhibit the rate of reaction of the oxidant with the organic substrate and/or inhibit the ability of the spent oxidant to be regenerated.
Thallium is a known oxidizing agent which has the potential of presenting an excellent two electron oxidant but has not been previously used to an extensive degree or on an industrial scale because of the inability of both the thallium(I) and thallium(III) species to be maintained in solution at high concentrations and due to the difficulty of generating the thallic oxidant in a simple and effective manner. For example, U.S. Pat. No. 3,048,636 teaches the use of low concentrations of thallium sulfate in order to avoid precipitation of either thallium(III) oxides, thallium(I) sulfate or complexes formed from thallium(I) and (III) sulfate. Thallium sulfates are generally restricted to low concentrations or must be used as a slurry. Both conditions are associated with poor reactivity and selectivity. One of the few thallium salts which exhibits high solubility is thallium(III) perchlorate. However, a potentially explosive situation is formed when the perchlorate anions are placed in contact with organic compounds.
The thallium salts are prohibitively expensive and must, therefore, be capable of being stable, react with the organic substrate cleanly and be easily regenerated to its higher valence state. This requires the thallium(III) salt to exhibit a high degree of stability and solubility in the reaction medium and be capable of achieving good reaction rates. In addition, the thallium(I) ion must also be highly soluble to be capable of being regenerated to the thallium(III) ion under conditions of high current efficiency at the anodic portion of the electrochemical cell. It has heretofore been believed that thallium must be used under a very narrow set of conditions or under inefficient conditions which could not demonstrate the potential necessary to provide an effective industrially suitable process.
Various processes are known to generate thallic ions from thallous ions but the majority of them are either expensive to do, require additional oxidant which precludes the systems use in providing a clean organic synthesis, causes accumulation of undesirable by-products, has a low efficiency of ability to generate the thallic ion or a combination of these defects. For example, chemical oxidation of thallous is, of course, possible with the very powerful agents such as chlorine gas and aqua regia, but these materials are objectionable as being somewhat difficult to handle (requiring expensive low-corrosion equipment), and cause the accumulation of undesirable materials in the system.
Hirose et al., in U.S. Pat. No. 3,399,956, report a system for oxidizing thallium with oxygen, which involves an acidic aqueous medium containing chloride or bromide and an ion of a "redox metal" such as copper or iron. In U.S. Pat. No. 3,479,262, MacLean et al describe a process to oxidize an olefin using thallium. The thallic ion is regenerated by a noble metal catalyzed oxidation using cerium(IV) as the oxidizing agent.
Other systems for oxidizing thallium are described in U.S. Pat. Nos. 3,486,992 to Frye, 3,759,804 to LeBris et al., 4,031,196 to Lenard, 4,115,420 to Brill, 4,115,421 to Brill, 4,058,592 to Rizkalla, 4,371,431 to Switzer et al as well as other methods.
Each of the above processes of forming thallium(III) has one or more disadvantage which makes it inappropriate for use in being an effective oxidant for organic compounds. In most instances either the thallous or the thallic specie is insoluble in the reaction medium. When the thallous specie is insoluble it impedes the separation and recovery of the organic product as well as lowering the effective oxidation to thallic ions. When the thallic specie is of low solubility, it reduces its effectiveness as an oxidant for the organic substrate.
It must be understood that although thallous/thallic ions have been known and used in oxidation reactions, there is a need to have a system wherein the thallic oxidant can be sufficiently stable under oxidizing conditions to be useful in indirect electrochemical processes, to be capable of undergoing repeated cycling between its thallous (Tl.sup.+1) and thallic (Tl.sup.+3) species in a high degree of efficiency under the reaction and electrolysis conditions, to be capable of exhibiting high reaction rates to make the process attractive on a commercial scale, to have high solubility to aid in the efficiency of the reaction and to eliminate the problems associated with slurries of thallium salts. It is readily seen that a means of achieving this combination of desired properties would aid in providing a process which would find a high degree of acceptance in electrochemical oxidation of organic compounds.